Observation and analysis of whistler mode echoes received by RPI on IMAGE at high latitudes

Abstract

Whistler-mode wave-injection experiments with Radio Plasma Imager (RPI) on IMAGE offer an opportunity to observe the whistler-mode echoes. We have performed raytracing studies to investigate accessibility of whistler-mode waves injected from IMAGE to various regions of the magnetosphere and also to other satellites such as Akebono. RPI detected both discrete and diffuse whistler-mode echoes during our observing period (April 21 to August 28, 2000) when IMAGE was at a low altitude (1̃000-7000 km) and mid-to-high latitudes (>25 - 40S̊) near its perigee. We believe that the discrete echoes are the result of RPI signals reflected at the Earth-ionosphere boundary and the diffuse echoes are the result of scattering of RPI signals by meter-scale irregularities. Raytracing analysis shows that both ducted and nonducted ray propagation are needed to explain the observed whistler-mode dispersion. Comparison of electron densities obtained from our raytracing analysis of dispersion with the electron densities obtained by Kletzing et al. in the Auroral Zone shows that these density values deduced from RPI data were about ten times higher. This may be because the antenna radiation efficiency is higher at higher electron densities.